Discussion on Supercapacitor Charger Balancing

Linear Technology Corporation introduces the LTC4425, the latest addition to its two-cell supercapacitor charger family, which uses a thermally limited linear constant current - constant voltage (CC-CV) architecture to charge two series supercapacitors to a programmable output voltage from a Li-Ion/Polymer battery, USB port or other current limited power source from 2.7V to 5.5V.

The LTC4425 has two modes of operation: Charge Current Profile (Typical) mode and LDO mode. In Charge Current Profile mode, the device charges the top of the supercapacitor stack to the input voltage VIN, using a charge current that varies inversely with the input-to-output voltage difference to prevent excessive heating. LDO mode charges the supercapacitor stack to an externally set output voltage, using a fixed, externally programmable charge current. The charge current is resistor programmable up to 2A (3A peak), and each capacitor is protected against overvoltage by an internal shunt (selectable 2.45V/2.7V). The LTC4425's built-in current-limited ideal diode has an extremely low 50mΩ on-resistance to prevent VIN from driving backward, making the device ideal for a wide range of high peak power battery and USB-powered devices, industrial PDAs, portable instrumentation and monitoring equipment, power meters, supercapacitor backup circuits, and PC Card/USB modems.

Table 1: Comparison of supercapacitors, ordinary capacitors and batteries

Supercapacitor (supercapacitor, ultracapacitor), also known as double-layer capacitor (Electrical Doule-Layer Capacitor), electrochemical capacitor (Electrochemcial Capacitor, EC), gold capacitor, farad capacitor, stores energy through polarized electrolyte. It is an electrochemical element, but no chemical reaction occurs during its energy storage process. This energy storage process is reversible, and it is precisely because of this that supercapacitors can be repeatedly charged and discharged hundreds of thousands of times. Supercapacitors can be regarded as two non-reactive porous electrode plates suspended in an electrolyte. When electricity is applied to the plates, the positive plate attracts negative ions in the electrolyte, and the negative plate attracts positive ions, actually forming two capacitive storage layers, with the separated positive ions near the negative plate and the negative ions near the positive plate.

Summary - Supercapacitors vs Batteries:

Battery:

High energy density

Moderate power density

Large equivalent series resistance (ESR) at low temperatures

Supercapacitors:

Moderate energy density

High power density

Low ESR (even at low temperatures)

(About 2 times higher from -20°C to 25°C)

Limitations of Supercapacitors:

Each cell is limited to a maximum of 2.5V or 2.75V

In stacked applications the difference in leakage current must be compensated

At high charging voltages and high temperatures, the lifespan is shortened more rapidly

Earlier generation two-cell supercapacitor chargers were designed for low current charging from 3.3V, 3-cell AA or Li-ion/Polymer batteries because these ICs used a boost topology. However, improvements in supercapacitor technology have expanded the market to include many medium to higher current applications that are not necessarily limited to consumer electronics. Key applications include high current portable electronic devices such as solid state disk drives and mass storage backup systems, industrial PDAs and convenience terminals, data loggers, instrumentation, medical equipment, and a variety of "last minute" industrial applications.

Design Challenges of Supercapacitor Chargers

Supercapacitors have many advantages, but when two or more capacitors are stacked in series, designers face issues such as capacity balancing, overvoltage damage to the capacitor when charging, drawing too much current, and large board area/solution. If frequent large peak power bursts are required, then a larger charging current may be required. In addition, many charging sources may be current limited, for example, in battery buffer applications or in USB/PCCARD environments. For space-constrained, high-power portable electronic devices, it is critical to deal with these situations.

Balancing the capacitance of supercapacitors connected in series ensures that the voltage on each capacitor is approximately equal, while the lack of capacitance balance of supercapacitors may cause overvoltage damage. For low-current applications, charge pumps use external circuits that match each capacitor with a balancing resistor, which is an inexpensive solution to this problem. As explained below, the value of the balancing resistor will depend mainly on the leakage current of the capacitor. However, if the leakage current between the series capacitors is mismatched, the capacitors may overvoltage at the beginning of recharging unless the designer selects a balancing resistor that can provide a load current on each capacitor that is much larger than the capacitor leakage current itself. Balancing resistors cause unnecessary components and permanent discharge currents, which burden the application circuit. If the mismatched capacitors are charged with large currents, they also do not provide overvoltage protection for each capacitor.

就中到较大功率应用而言，另一个可解决超级电容器充电问题而且不算昂贵的方法是，采用一个电流受限的开关加分立器件和外部无源组件。采用这种方法时，电流受限的开关提供了充电电流和电流限制，同时电压基准和比较器 IC 提供电压箝位，最后，具平衡电阻器的运放实现超级电容器的容量平衡。然而，镇流电阻器的值越低，静态电流越高，电池运行时间越短，显然的好处是节省了费用。不过，这种解决方案实现起来非常笨重，而且性能充其量也就是略微好一点。

上述满足超级电容器充电器 IC 设计限制的任何解决方案都必须与一个大电流充电器相结合，以用于具自动容量平衡和电压箝位的两节串联超级电容器。因此，凌力尔特公司开发了一款面向中到大功率应用的简单但先进的单片超级电容器充电器 IC，该 IC 无需电感器、无需平衡电阻器、有各种工作模式并具有低静态电流。

A simple solution

The LTC4425's automatic energy balancing function maintains equal voltages on both supercapacitors, eliminating the need for balancing resistors while protecting each supercapacitor from overvoltage damage and minimizing capacitor leakage current. The IC operates at a very low 20uA quiescent current when the output voltage is in regulation, and draws only 2uA from VIN or VOUT (whichever voltage is higher) when shut down. The basic charging circuit requires only six external components and is highly compact, with a tiny package that occupies 9mm2 and a leaded package. Other key features include a VIN power fault indicator and continuous monitoring of the VIN to VOUT current through the PROG pin. Other protection features include current and thermal limiting, which reduces the charging current in the event of excessive temperature.

The LTC4425 is a new device in Linear Technology's two-cell supercapacitor charger family for high peak power, data backup and "last minute" applications in portable and data storage applications. The device uses a linear constant current, constant voltage architecture with thermal limiting to charge two series supercapacitors to a programmable output voltage from a Li-Ion/Polymer battery, USB port or 2.7V to 5.5V current limited power supply. The LTC4425 has two operating modes: charge current profile (normal) mode and LDO mode. The charge current can be programmed to 2A (3A peak) with a resistor, and each capacitor is protected from overvoltage damage by an internal shunt. The IC's internal current-limited ideal diode has a very low 50mΩ on-resistance to prevent VIN from back-driving and make the device suitable for a variety of high peak power battery and USB powered devices, industrial PDAs, portable instrumentation and monitoring equipment, power meters, supercapacitor backup circuits, and PC Card/USB modems.

The LTC4425 is available in two compact, thermally enhanced packages: a 12-lead, low profile (0.75mm height) 3mm x 3mm DFN package; and a 12-lead MSOP package. The device operates over the -40°C to 125°C junction temperature range.

Figure 1: LTC4425 Block Diagram/Application Circuit LDO Mode

An LDO is a linear regulator. A linear regulator uses a transistor or FET operating in its linear region to subtract excess voltage from the applied input voltage to produce a regulated output voltage. The dropout voltage is the minimum difference between the input and output voltages required for the regulator to maintain the output voltage within 100mV of its rated value. Positive output voltage LDO (low dropout) regulators typically use a power transistor (also called a pass device) as a PNP. This transistor allows saturation, so the regulator can have a very low dropout voltage, usually around 200mV, compared to a traditional linear regulator using an NPN compound power transistor with a dropout voltage of around 2V. A negative output LDO uses an NPN as its pass device and operates similarly to a positive output LDO's PNP device.

In LDO mode, the output voltage (VOUT) is set through the FB pin with an external resistor divider network consisting of RFB1 and RFB2, while the charge current is set through the PROG pin with an external resistor RPROG. See the block diagram shown in Figure 2. The charger control circuit consists of a constant current amplifier and a constant voltage amplifier. When the IC is started to charge a discharged supercapacitor stack, initially the constant current amplifier takes control and servos the PROG pin voltage to 1V. The current through the PROG resistor is multiplied by the ratio of the sense MOSFET (MPSNS) and the power MOSFET (MPSW), which is approximately 1,000, to charge the supercapacitor stack. When the output voltage VOUT approaches the set value, the constant voltage amplifier takes over control and reduces the charge current if necessary to keep the FB pin voltage equal to an internal reference voltage of 1.2V. Because the PROG pin current is always approximately 1/1,000 of the charge current, the PROG pin voltage continues to indicate the actual charge current, even when the constant-voltage amplifier is in control.

Charging Current Curve (Typical) Mode

When the FB pin is shorted to the input voltage VIN, the LTC4425 enters the charge current curve mode. In this mode of operation, the constant voltage amplifier is internally disabled, but the charge current is still set by the external RPROG resistor. If the input-to-output voltage difference (VIN – VOUT) exceeds 750mV, the charger provides 1/10 of the programmed charge current to limit power dissipation within the chip. As VOUT rises further, the voltage across the charger FET becomes too low to support the full charge current. Therefore, the charge current is gradually reduced and the charger FET enters the triode (Ohm’s law) operating region (see Figure 3). Since the charger FET RDS(ON) is approximately 50mΩ, when the charge current is set to 2A, the FET will enter the Ohm’s law region and the charge current will begin to decrease when VOUT is within approximately 100mV of VIN.

电压箝位电路

The LTC4425 is equipped with circuitry that limits the voltage across the two supercapacitors in the supercapacitor stack to the maximum allowable voltage, VCLAMP. There are two VCLAMP preset voltages selectable through the SEL pin: 2.45V or 2.7V. For the lower 2.45V VCLAMP voltage, the SEL pin should be set to a logic low, while for the higher 2.7V VCLAMP voltage, the pin should be set to a logic high. Similarly, if the voltage across the top capacitor (VTOP) reaches VCLAMP first, the PMOS shunt transistor turns on and begins to discharge charge from the top capacitor to the bottom capacitor.

When the voltage across either supercapacitor reaches within 50mV of VCLAMP, the transconductance amplifier begins to linearly reduce the charge current. By the time either shunt device is turned on, the charge current has dropped to 1/10 of the set value and remains at that value as long as the shunt device is turned on. This is to prevent the shunt device from being damaged by excessive heat. The comparator controlling the shunt device has 50mV of hysteresis, which means that when the voltage across either capacitor drops by 50mV, the shunt device is disconnected and resumes normal charging at the full charge current unless limited by the other amplifier controlling the gate of the charger FET. If both capacitors exceed their maximum allowable voltage, VCLAMP, the main charger FET is completely turned off and both shunt devices are turned on. The two shunt devices are actually current mirrors, ensuring that a larger current is divided than the current through the charger FET.

Leakage current balancing circuit

The LTC4425 is also equipped with an internal leakage current balancing amplifier (LBA) that makes the midpoint (i.e. VMID pin) voltage exactly equal to half of the output voltage VOUT. Due to its limited 1mA source and sink capability, it is designed to handle slight mismatches of supercapacitors caused by leakage currents, rather than to correct any severe mismatches caused by defects. The balancer operates as long as the input voltage is present. With this internal balancer, no external balancing resistors are required.

Table 2 compares the devices in Linear Technology's supercapacitor charger family.

in conclusion

Supercapacitors are now being used in applications once dominated by batteries. Initial applications were for low currents, but technology has advanced and supercapacitors are now used in a variety of medium to high power applications in both consumer and non-consumer markets. Supercapacitors have many inherent advantages over batteries, such as providing higher peak power, longer cycle life, and smaller form factors. However, product designers using supercapacitors face many issues, such as the need for capacity balancing and potential overvoltage damage to supercapacitors.